1. Field of the Invention
This invention relates to a key pad suitable as a push-button switch component to be incorporated in the input portion of an electronic apparatus, such as a telephone, an audio apparatus, a television set, a VCR, a facsimile apparatus, a copying machine, or a vehicle-mounted apparatus, and, in particular, to a key pad, which is suitable as the push-button switch component of a portable apparatus of which a reduction in thickness and size is required. Further, this invention relates to a resin key top injection mold and a resin key top manufacturing method.
2. Related Background Art
Up to now, an illumination type key pad equipped with a translucent elastic key sheet and a translucent resin key top has been widely used as a push-button switch component to be incorporated in the input portion of a portable apparatus, such as a mobile phone. FIG. 12 shows a specific example of such a key pad. In the drawing, a key pad 1 is composed of a key sheet 2 obtained through molding of translucent silicone rubber or translucent thermoplastic elastomer into a sheet, and resin key tops 3 obtained through injection molding of a translucent thermoplastic resin into buttons and firmly attached to the key sheet 2 through the intermediation of translucent adhesive layers 4. This key pad 1 is placed on a substrate surface 7a having thereon disc springs 5, chip components 6 with LEDs or the like serving as the illumination sources, etc., and is integrated with a substrate 7, etc. to form a push-button switch for input operation.
In many cases, each resin key top 3 in such a push-button switch has at the outer periphery of the lower end of its side surface an outwardly protruding flange portion 8. This flange portion 8 is adapted to engage with the back surface of a casing 9 to prevent the key top from being detached from the apparatus (casing 9). Further, when the LEDs or the like incorporated in the chip components 6 are caused to emit light to illuminate the resin key tops from within the casing 9, leakage of light through the gaps between the resin key tops 3 and the casing 9 can be prevented by equipping the flange portions 8 with shielding layers, thereby enhancing the illumination property for the resin key tops 3.
Generally speaking, there is a requirement for a further reduction in the thickness and size of portable apparatuses, such as mobile phones. In view of this, in the key pad 1 as shown in FIG. 12, a further reduction in the thickness and size of the components including the key sheet 2, the resin key tops 3, and the adhesive layers 4 is being contemplated. However, a further reduction in the thickness and size of the key pad would result in a reduction in the interval between the resin key tops 3 facing each other and a reduction in the vertical distance between the resin key tops 3 and the substrate 7. Thus, it has been concluded that the flange portions 8 of the resin key tops 3 would inevitably come into contact with the chip components 6 protruding from the substrate surface 7a. 
It might be possible to avoid this problem of contact, for example, by reducing the number of chip components 6. However, that would result in some of the resin key tops 3 not being sufficiently illuminated, making it impossible to uniformly and sufficiently illuminate all the resin key tops 3. It might also be possible to reduce the size of the resin key tops 3. However, that would involve a deterioration in the depression-operability of the resin key tops 3, resulting in a rather inaccurate input operation, which is inconvenient for the user.
In this regard, the present inventor considered a method of partially trimming the flange portions 8 of the resin key tops 3. However, as shown in FIG. 13, with the method in which the flange portions 8 are partially trimmed by a trimming die after the molding of the resin key tops 3, the cutting edges a through c are likely to involve positional variation, so that the width dimensions of the remaining flange portions 8 are rather varied, resulting in frequent occurrence of light leakage at the time of illumination. In particular, the resin key tops of a portable apparatus which is generally very small in total size are subject to such variation in the cutting edges a through c.